A tappet of the pre-cited type is known from DE-OS 196 22 174. FIG. 8 of this document, for example, discloses a tappet having a coupling mechanism comprising in its annular section, two slides configured as pistons. Each of these pistons can be displaced toward the circular section by hydraulic means acting axially from the outside so that the entire tappet follows a lift of the cams which act on the annular section. To exclude switching errors, the outer cams possess a control contour that cooperates with locking means (balls) on the outer peripheral surface of the pistons. These pistons comprise axially spaced annular grooves. At the end of a base circle phase of the cams, the balls are displaced into these grooves by the control contour. An axial movement of the pistons is thus prevented.
A drawback of this prior art is that the locking means are situated in the region of the hydraulic medium supply to the pistons. This can result in hydraulic medium losses or make it problematic to implement sealing measures in this region. At the same time, due to their stiffness, these balls can get seated on an annular shoulder between the annular grooves of the pistons when a command to lock has been triggered by the control contour. This occurs when the pistons are not sufficiently displaced and can lead to a loading or even destruction of the component.